avcodec/vvcdec: schedule next stage only if the current stage reports no error

If the current stage reports an error, some variables may not be correctly initialized.
Scheduling the next stage could lead to the use of uninitialized variables.

libavcodec/vvc/thread.c

@@ -656,9 +656,9 @@ static void task_run_stage(VVCTask *t, VVCContext *s, VVCLocalContext *lc)
                 "frame %5d, %s(%3d, %3d) failed with %d\r\n",
                 (int)fc->decode_order, task_name[stage], t->rx, t->ry, ret);
         }
-    }
-
+        if (!ret)
             task_stage_done(t, s);
+    }
     return;
 }